The applicant of the present application has proposed a device for measuring brightness of gemstones which measures glitter of gemstones glittering upon receiving natural light in an objective manner in Japanese Patent Application No. 2010-119349 filed on May 25, 2010. The background art of the device for measuring brightness of gemstones will be explained below first by citing the contents of this application which is incorporated herein.
Patent Document 1 discloses an example of the device for measuring brightness of gemstones, which is shown in FIG. 14 of the present application. FIG. 14 is a perspective view of an appearance of the device for measuring brightness of gemstones disclosed in Patent Document 1, which is a background art of the device for measuring brightness of gemstones of the present invention.
This device for measuring brightness of gemstones 60 has diamond as a measuring object. At the center of a transparent glass circular disk 51b, a diamond is placed on the table with its crown being contact with the transparent glass surface, and then covered by a hemispherical dome 51a having a white interior surface. By moving an annular light source 52 up and down right below the glass circular disk 51b, incident light angle from the crown side can be varied. By arranging a detector 55, which is a CCD camera, below the annular light source, only the scattered light essentially vertical with respect to the crown table is measured as bright dot in the field of view.
With this device 60, the crown is set at the bottom, while the pavilion is set at the top. The angle of incident light from the crown side is changed with the annular light source 52 moving up and down. Light intensity is measured such that incident intensity for each incident angle (i.e. depending on the change of the height of the annular light source) is detected by the detector 55 arranged immediately below the table surface on the vertical axis, and then accumulated. Scattered light rays in a dispersed manner from the pavilion side are bounced by a white hemispherical dome 51a and reenter. Among them, light rays entering into the detector from the direction of the normal line of the table side is also accumulated as “glitter” into the light intensity value.
Therefore, the device 60 cannot evaluate the size of the bright dots (i.e. solid angles of the scattered light rays), but can only count the number of the intense scattered light rays entering into the field of view. Consequently, small bright dots (scattered light with a small solid angle) are overestimated due to the great number of count, while scattered light with a larger solid angle which comes from larger facets is underestimated.
Since brightness sensible to human eyes depends on the size of bright dots (i.e., size of the reflecting surface, facet), even though the entire amount of scattered light intensity is the same, a diamond “with bright dots each of which has a large reflective solid angle by scattered light and which are small in number” gives greater aesthetic impression. On the contrary, a diamond “with bright dots each of which has a small reflective solid angle by scattered light and which are large in number” is not so attractive as a glitter sensible to human-eyes, which is, however, evaluated as a “specimen releasing great glitter” only due to a large number of count and the total light intensity of the scattered light.
Furthermore, in this device 60, the central axis of the glass circular disk 51b and the axis of the detector 55, i.e., a CCD camera, coincides with each other, and the light source 52 is arranged annularly at a symmetrical position. This arrangement is assumingly selected in order to prevent intense light, i.e., the most intense reflecting light off the table surface, from entering into the detector 55. This arrangement, however, does not necessary reproduce light incidence and scattering under an actual situation of use.
In other words, the measurement method of this device 60, (i.e., a measurement method for counting “scattered light exiting from the direction essentially normal to the table surface” by means of “light incident from the direction other than that normal line to the table surface”), should be regarded as a measurement under a condition different from actual situation of use, also in view of the conditions of light incidence and scattering.
Since both incident light and scattered light (i.e. light recognized to human eyes as “glitter”) do not always come from “the direction normal to table surface,” under an actual situation of use, for example, the disclosed device model does not simulate an actual situation of use. It is necessary to measure light incidence from any angle direction, and light scattering to any angle direction in order to simulate and quantify an actual case of use.
The above-mentioned problem is partially solved by a device disclosed in Patent Document 2, where a hole is provided at the top of a paraboloidal mirror, and then a measuring object is placed on the focal point of the paraboloidal mirror which is assumingly located near the top. At least two collimated light rays parallel to the central axis of the paraboloidal mirror are irradiated from the paraboloidal mirror side. They are reflected by the paraboloidal mirror, and are irradiated onto the measuring object being located on the focal point. By thus reflected light off the object, bidirectional reflectance distribution function (BRDF) and bidirectional transmittance distribution function (BTDF) can be measured.
Patent Document 2 indeed discloses that light passing through the focal point is parallel to the central axis of the paraboloidal surface of the paraboloidal mirror, and that it also passes through the focal point which reflects the light parallel to the central axis of the paraboloidal surface. But the document does not mention size of the solid angle, the number and the like of light which is necessary for evaluating brightness of a gemstone.
In addition, measurement precision may be affected in some paraboloidal surfaces, depending on their shapes when the relationship between the position of the central axis of the surrounding portions and the reflecting angle is too tight. This possibility is not mentioned in the Patent Document 2, either.
In the above-mentioned Japanese Patent Application No. 2010-119349, a device for measuring brightness of gemstones for solving the above-mentioned object has been proposed. It was found afterwards that this configuration of the device using a paraboloidal surface should not be limitedly used for gemstones but can be applied to a device for measuring properties of scatterers where properties of scatterers are to be measured from a stereoscopic scattering distribution when the scatterers are exposed to electromagnetic waves with a certain wavelength distribution, and also to a color measuring device for scattered light of gemstones where this device for measuring properties of scatterers is used for measuring color of scattered light of the gemstones and further to a device for measuring luminescence distribution of a self-luminescent illuminant.